1. Field of the Invention
The present invention relates to a chip-type filter that includes a chip-type solid electrolytic capacitor that contains conductive polymer as solid electrolyte and can be surface-mounted, and forms a π-type filter.
2. Background Art
In response to the increase in frequency of an electronic device, a capacitor as one of electronic components has been also demanded to have an impedance characteristic in a region of a frequency higher than that in a conventional capacitor. In order to reply such a demand, various solid electrolytic capacitors containing conductive polymer of high electric conductivity as solid electrolyte have been studied.
Recently, a solid electrolytic capacitor used in a periphery of a central processing unit (CPU) of a personal computer has been strongly demanded to be downsized and have large capacitance. Equivalent series resistance has been strongly demanded to be reduced (ESR reduction) in response to the increase in frequency, and equivalent series inductance has been strongly demanded to be reduced (ESL reduction) in order to reject noise and improve the transient responsibility. Various studies are performed in order to reply such demands.
FIG. 8A through FIG. 8D are a plan sectional view, a front sectional view, a bottom sectional view, and a bottom view showing a structure of a conventional chip-type solid electrolytic capacitor, respectively. Each of capacitor elements 21 has a positive electrode body (not shown) made of valve action metal. The surface of the positive electrode body is roughened, and then a dielectric oxide layer (not shown) is formed on the surface. An insulating section (not shown) is disposed at a predetermined position of the positive electrode body, and the positive electrode body is divided into positive electrode section 22 and a negative electrode-forming section (not shown). A solid electrolyte layer made of conductive polymer and a negative electrode layer made of carbon and silver paste (all are not shown) are sequentially formed in a stacking manner on the dielectric oxide layer of the negative electrode-forming section, thereby forming negative electrode section 23. Capacitor element 21 is formed in a flat plate shape.
Laminated body 24 is formed by stacking a plurality of capacitor elements 21 so that positive electrode sections 22 of capacitor elements 21 are disposed in the alternately opposite directions. Positive electrode lead frame 25 integrally joins positive electrode sections 22 in laminated body 24. Negative electrode lead frame 26 integrally joins negative electrode sections 23 in laminated body 24.
Positive electrode lead frame 25 is joined to the upper surface of positive electrode terminal 27. Thin sections 27B are disposed at both ends of the width direction in positive electrode terminal 27, and the center part other than thin sections 27B defines positive electrode terminal section 27A during mounting. Negative electrode lead frame 26 is joined to the upper surface of negative electrode terminal 28. Thin section 28B is disposed in a central part of the width direction in negative electrode terminal 28, and the both end parts other than thin section 28B define negative electrode terminal sections 28A during mounting.
Insulating outer resin 29 integrally covers laminated body 24, positive electrode lead frames 25, negative electrode lead frame 26, positive electrode terminals 27, and negative electrode terminals 28. Thin sections 27B and 28B respectively disposed in positive electrode terminals 27 and negative electrode terminals 28 are also integrally covered with outer resin 29. Positive electrode terminal sections 27A are disposed oppositely at two positions and negative electrode terminal sections 28A are disposed oppositely at two positions in the exposed state on the lower surface as the mounting surface of the chip-type solid electrolytic capacitor. In other words, this chip-type solid electrolytic capacitor has a four-terminal structure.
In the structured chip-type solid electrolytic capacitor, thanks to its four-terminal structure, magnetic fluxes generated by currents flowing between the terminals cancel each other. Therefore, the ESL can be significantly reduced. When the distance between the terminals is made as short as possible to decrease the loop area, the ESL can be further reduced.
However, even when such a solid electrolytic capacitor is used, in a π-type filter formed by connecting an external inductor element to it, extra resistance component or extra inductance component occurs in the connection path. As a result, the impedance increases.